1. Field of the Invention
This invention relates in general to certain new and useful improvements in wide bandwith video amplifier circuits and more particularly, to wide bandwith video amplifiers having high tracking linearity and adjustable DC level of the output signal relative to a true ground potential.
2. Brief Description of the Prior Art
In recent years, there has become an increased need, and an increased demand for high resolution video monitors, projectors and projection systems for purposes of generating computer graphics and alpha-numeric displays, and for high resolution in other types of CRT monitors where wide bandwith and/or high tracking linearity is needed.
Generally, most conventional video amplifiers which are used for amplifying video signals have a number of circuit components and stages which provide various functions necessary for proper amplification of a video signal and display thereof. A video signal is normally introduced into an AC coupled pre-amplifier-gain control stage. This AC pre-amplifier is operated generally from a low DC voltage supply.
The amplified video signal is then introduced into a DC restorer and then into an output stage of the conventional video amplifier. The output stage sometimes includes a buffer section. The output stage also typically includes a feedback to a summing node, which is connected to the input of the DC restorer amplifier. When the video signal is passed through a capacitor, either at the AC amplifier stage or thereafter, the DC bias of this signal has been effectively removed. The feedback from the output stage to a summing node at the input of the DC amplifier adds the DC component during a predetermined period of time to the AC signal which had the DC bias stripped therefrom. This action clamps the video signal to a specific DC level, thus allowing proper operation of a cathode ray tube.
For low and mid-frequency amplifiers, this type of amplifier configuration operates quite effectively, inasmuch as the low frequency transistors can be made to withstand high breakdown voltages, as for example, from +90 volts to about +230 volts. However, presently available high frequency transistors are not capable of effectively withstanding a high voltage and therefore can not be used in the construction of high bandwith video amplifiers. High frequency transistors operate at lower collector-emitter voltages and have a lower collector-emitter breakdown voltage. Thus, in a video amplifier it would be desirable to have a high fequency transistor with a high collector-emitter breakdown voltage and this heretofore has been unavailable.
Another characteristic of most transistors is the limitation of the amount of power which can be dissipated by the transistor. At high voltage between the collector and the emitter, the transistor will dissipate more power, which may exceed safe operating conditions. In addition, the highest frequency of a video amplifier depends on the value of the collector resistor (R.sub.c) and the load and is given by ##EQU1## where C.sub.LOAD is the capacitance of the load and f.sub.H is the highest frequency of the amplifier.
The load is usually predetermined by the type of transistors and the CRT involved. Therefore, to increase the bandwith of the amplifier the collector resitance must be decreased, which in turn, increases the power that must be dissipated by transistor. In effect, since P=I.sup.2 /R.sub.c there is a technical dilemna in that high collector-emitter voltage is needed to properly bias the cathode tube, but the transistor can not properly and efficiently operate at high collector-emitter voltages because of a high power dissipation. To decrease the power dissipation the collector resistor voltage must be increased, which, in turn, degrades performance of the amplifier at high frequencies.
Another problem with color video amplifiers is that each gun of the video display screen is operated by an individual amplifier. Thus, for a three color picture tube including, for example, red, green and blue guns, three individual amplifiers are required. Each amplifier should ideally have the same AC characteristics and the same linearity, although each should have an independent DC bias adjustment of the output signal relative to true ground potential.
In actual operation, each gun of a multi-color video tube has significantly different characteristics in that there has never been any perfected technique for making identical guns. This is clearly apparent from the graph of FIG. 2, which is hereinafter described in more detail. Accordingly, each gun of a picture tube requires a different DC voltage potential in order to operate properly. For example, one gun may operate properly at an 75 volt level and another gun may operate properly at a 110 volt level. Therefore, each amplifier must be biased in a different manner in order to provide the different DC voltages relative to a true ground potential on each of the guns. However, the linearity and frequency characteristics of each amplifier depend on the DC bias of that amplifier. This creates another technical dilema in that to provide the same linearity and AC characteristics all amplifiers require an equal DC bias. However, for proper operation of a multi-gun tube, each amplifier must have a different DC bias. Heretofore, there has not been any effective resolution of this problem.
U.S. Pat. No. 3,281,705 to Frye discloses a wide band signal invertor circuit which uses separate signal paths for high frequency signal portions and low frequency signal portions. This patent discloses a separate potentiometer for controlling a DC voltage level on an output, as well as a source of negative DC bias voltage through a fixed bias resistor, and a DC negative bias voltage through a variable resistor to vary the DC voltage level at an output terminal.
U.S. Pat. No. 3,413,563 to Tongue discloses a transistor amplifier having substantially disipationless feedback paths to achieve a wide band response with substantially uniform gain. U.S. Pat. No. 3,461,393 to Webb, Administrator with respect to George D. Thompson et al, discloses a broad band distribution amplifier having a thermistor in the input stage of the amplifier to receive high frequency RF input signals and which thermistor is variable.
U.S. Pat. No. 3,810,256 to Van Doorn discloses a broad-band transistor amplifier having a conventional negative feedback circuit which utilizes a cascade arrangement of two impedenances and injects distortion products in phase opposition into the output circuit of the amplifier.